6g15/vibroverb single channel hybrid reverb feedback with hi input

[Jatchley]

Hey y’all. So I built this amp sluckey drew up for me. I switched to a gz34 for higher voltages because the 5u4 just had too much sag and was pulling down the b+ too much. But I also added a HI/LO input and when I use the high (1M) the reverb feedbacks very soon. When I use the lo input I can crank my heart out and get very surfy. Other than this problem the amp is dead on voltages for the schematic for the 6g15 and within 20% for the rest of the amp. I think there’s impedance thing happening and I’d love y’allS input on how to fix my inputs.

[pdf64]

What exactly do you mean by ‘the reverb feedbacks’?
eg howl from the reverb tank itself? If so, does turning the volume on the guitar down stop it?
What controls on the amp affect the issue?
A video demonstrating the issue may be very beneficial.

[ac427v]

Don't know about the feedback issue.
Your schematic shows two 80uf caps in parallel for the first node of the power supply. If you built it that way there may be lots of smoke coming soon. That would mean 160uf of capacitance for that poor rectifier tube to charge in an instant. Or am I reading the small print wrong?

[sluckey]

Your schematic shows two 80uf caps in parallel for the first node of the power supply.

The original schematic I gave him used two 20µF.

Jatchley, can you control the feedback with the Dwell pot? The schematic I provided only had one input and it was "HIGH". We need to see some gut shots of this amp.

[Jatchley]

Yes dwell controls it and Reverb tone changes the pitch of it (or kind of a filter)  I set up the power section Kind of like a super reverb. So the 80uf on the schematic should be in series. But other than the using the HIGH input and the reverb feeding back. The amp sounds great. Low noise classic fender tone. The bias vari trem is nice and deep even at slowest settings. And the reverb operates perfectly if I use the Low input. I’ll get a video of it later today.

[sluckey]

So the 80uf on the schematic should be in series.

OK. But are they actually in series inside the amp? And if so, did you put a 220K/1W across each cap?

Are you using a guitar with hot or active pickups? The 6G15 was designed before there were any such hot pickups. Connect something like a stock strat or tele to the high input. Still squeal?

[sluckey]

Does the reverb squeal with nothing plugged into either input?

[pdf64]

As the Dwell control affects it, the feedback may be due to the valve in V1, rather than the reverb tank.

[Jatchley]

They are in series in real life yes. I use a jazzmaster with 500k pots so single coils are already being used. Here’s a slightly old DIYLC. There might be other mistakes on here that are now fixed in real life. Like the phase inverter input on the board. The Diy file has the mixing resistor miswired. But I fixed that. And no no squeal with nothing plugged in. It’s a very mid range note not super high whistle or sputtery just As soon as dwell hits around 2-3 I can get it up to 4-5 if the tone is all the way down.

[Jatchley]

Something tells me it’s the 1m to ground here. I’m also using shielded cable to and from the reverb section For those long runs to the pots as well.and using shielded from the volume pot to the preamp tube

[sluckey]

Something tells me it’s the 1m to ground here.

Well, measure resistance from V2 pin 2 to confirm. V2 may just have a bit too much gain. Disconnect the cap from V2 pin 3. Any better?

[Jatchley]

V2 pin 2 to the 1M is 1.03M

[Jatchley]

Woooo that was it. That cathode bypass cap was adding too much gain. Sounds good on both hi and lo now. Thanks!

[sluckey]

V2 pin 2 to the 1M is 1.03M

Really? Now measure V2-2 to GROUND. One probe on chassis. One probe on the tube socket.

Why do you suspect the 1M that's in the V4 circuit?

[Jatchley]

I suspected it. idk just mislogic I know it’s essentially an Input jack without the jack. But with the actual input jacks I thought it maybe was doing something weird. Removing that cap worked liked a charm.

[sluckey]

You have a unique amp. Enjoy it. Other Fender fans will be jealous. I had considered doing one like that a few years ago. Just never got a round tuit.

[Jatchley]

Oh yeah it sounds amazing. I have it paired with 2x10 Jensen p10qs run at 4ohms currently. I’m saving up for Celestion golds to see if they’ll be the cherry on top. I added a NFB lift, that mid cap stack switch and a raw switch. So the tonal flexibility is pretty high. Thanks so much for all your help in this.

[sluckey]

Still want to see some pics of this amp, inside and outside. Cab too if you have one.

[Jatchley]

Here’s a snap of the outside. And  pics of the guts and back of chassis from a few months back. I did mess up the tube placement of v2 and v3 so I have some wires crossing around there but I kept the grids from the plates and that eliminated all hum. You have to take out the big bottles to remove it from the headcab. And the reverb tank mounted to the top of the headcab prevent fingers from accessing the back of the control panel And when tubes are glowing they’re too hot to make anyone wanna go reaching in. It’s more of a burn risk than a shock risk like any other amp. I still plan on adding a rackmount chassis to go around the current chassis (Top,bottom,and sides) so I could Safely rackmount it later. I also run it open back but wanted a convertible cab.

[Jatchley]

Alright I’ve come across one more problem. When playing hard I’ve noticed I’m tending to clip the preamp. Around 5 on the volume pot. Sounds like paper ripping. The thick meaty distortion comes in around 8 on the dial. It’s very very subtle and seems to only happen with A strong transient attack. All my voltages are pretty much dead on classic fender schematics for both the 6g15 section and the vibroverb section. I’ve got -45 on the bias. 455-460 on the 6L6 plates. All the preamp plates are dead on. My PI cathode voltage is around 60v tho And I know it’s schematically 90v. So my thoughts are is the buffer of the 6g15 hitting my preamp too hard? I have the preamp input set up as a hi input 1M to ground and 68k to grid. I’m wondering if the resonant peak coming from the 6g15 output is just driving it a lil bit too hard. I’ve tried 100k grid blockers all around and removed them because they didn’t change anything. And I tried removing the buffer cathode bypass cap to no effect. I’ve tried removing the 6k6 to check if it was just a noisey reverb tube. I’ve tried lower gain tubes in most positions dealing with dry signal. I’ve swapped all tubes between 3 different sets that are known to be good. The amp is 99% perfect except this one very tiny Preamp distortion bug. I’m using single coils. Any ideas? My guesses are resonant peak clipping or my speakers are distorting before the power amp does or Could this audible sag?

[sluckey]

You circled three components on your schematic, C6, R13, and R14. Those components need to be in place. Are they?

There was a note on the original schematic I gave you that explains why.

BTW, your layout drawing does not resemble your actual board. What's up with that?

[Jatchley]

The Diylc was made Accidentally in reverse. I realized after doing it I made the diylc as if it was a top mount chassis not a laydown. Those resistors that are circled are there.

Edit:  Well not reversed but polarity flipped. On The diylc the tubes and knobs are flipped with the board. Other than that it should be 99% the same.

[Jatchley]

Maybe I’m just fooling myself? I tried to recreate it for a video for you and it’s not happening now. I swapped in the highest Headroom 6L6gcs I could find in my stash and moved the amp away from a wall. it’s seemingly unnoticeable. Even before it’d be hard to tell something was distorting. Clean tone was over powering it always. It’d only ever last just a second during the initial attack when strumming or picking aggressively Kinda near the edge of breakup. I just want this thing to be perfect. And it’s kinda silly of me to be like it’s turned up why is it distorting? With only a 40w OT and 2x10s my clean loud headroom might be lower than I was expecting.

[sluckey]

Divide and conquer. Temporarily disconnect the reverb unit from the amp and install a jack so you can go straight into the amplifier. What does that tell you?

[Jatchley]

Ok so I’ve taken the trem, and reverb entirely out of the equation by pulling the tube and re-wiring the inputs straight to the preamp. No difference with this gremlin. I replaced the MOD filter caps I used to F/T. I changed the PIs 220k resistors to 100k and switched to .02 coupling caps to the power tubes. No change. Here’s a question regarding those resistors circled. Would it be 220k to ground since we have a trem circuit in this amp? Or do I still account for a missing vactrol? I guess I’m asking what are the load differences between bias vari and the opto roach if any?

[sluckey]

Build it EXACTLY like the schematic I gave you and it should work.

[Jatchley]

It’s not that it doesn’t work. It does I’m just chasing the tiniest bit of what I think is blocking distortion. Even when it was exactly like the schematic I still had this tiny bit of blocking. And I’m just trying to find where it’s coming from.

[Jatchley]

Ok I put everything back to what the schematic says. Same problem. A tiny bit of distortion at about 5 on the dial that only really comes through with a hard attack on the strings.